For example, in a fixed communication system of a microwave band, communication may be carried out between two points on the surface of the sea level. At this time, a radio wave, transmitted by a transmission antenna and directly received by a reception antenna, is referred to as a direct wave; and a radio wave, transmitted by the transmission antenna and received by the reception antenna after being reflected on the sea level, is referred to as a reflected wave. The reception antenna receives a combination of the direct wave and the reflected wave.
The sea surface reflected wave is delayed due to the difference in the transmission path with respect to the direct wave. Thus the reflected wave becomes a signal having different amplitude/phase from those of the direct wave. Since the sea level is changed by a tide level or a wave, the path of the sea surface reflected wave is affected and changed. Accordingly, the reception antenna receives a combined wave of the direct wave and the sea level reflected wave whose amplitude and phase change with time. As a result, fading occurs by the changes in the sea surface.
A diversity scheme is suggested to overcome the fading problem.
In a fixed communication system of a microwave band, paths can be comparatively easily estimated. From such characteristics, it is possible to realize an antenna selection diversity in which antennas are installed in an inverse correlation.
The antenna selection diversity in which antennas are installed in an inverse correlation will be explained with reference to (a) to (c) of FIG. 5.
(a) of FIG. 5 shows the case in which communication is performed between point P and point Q. There are illustrated an antenna A in the point P, antennas B and B′ in the point Q, a reflection point R of a reflected wave which connects the antenna A and the antenna B, and a reflection point R′ of a reflected wave which connects the antenna A and the antenna B′.
(b) of FIG. 5 shows a path length LAB of a direct wave from the antenna A to the antenna B, and a difference ΔL (=(LAR+LRB)−LAB) with respect to a path length (LAR+LRB) of a reflected wave from the antenna A to the antenna B via the R point.
(c) of FIG. 5 shows a difference ΔL′ (=(LAR′+LR′B′)−LAB′) between a path length LAB′ of a direct wave from the antenna A to the antenna B′ and a path length (LAR′+LR′B′) of a reflected wave from the antenna A to the antenna B via the R′ point.
When communication is performed between the point P and the point Q in the example shown in (a) to (c) of FIG. 5, the difference in the path lengths between the direct wave and the reflected wave in the antennas B and B′ needs to be (2 m+1)λ/2 in order to satisfy the inverse correlation between the antennas B and B′ installed at the point Q. In other words, the condition for satisfying the inverse correlation is (ΔL−ΔL′=(2 m+1)λ/2).
λ is a wavelength of a signal, and m is an integer greater than or equal to 0.
FIG. 6 shows an example of reception levels of the antennas B and B′ in the inverse correlation. The reception levels of the antennas in the inverse correlation change as illustrated in FIG. 6.
In the graph shown in FIG. 6, the horizontal axis indicates a carrier phase [deg] of a reflected wave in the antenna B on the assumption that a carrier phase of a direct wave in the antenna B is 0, and the vertical axis indicates a level [dB]. FIG. 6 shows the received power (reception level) of the antenna B and the received power (reception level) of the antenna B′.
When the antennas are installed in an inverse correlation, if the reception level of one antenna is increased by changes in the sea surface, the reception level of the other antenna is decreased. At this time, the reception levels of both antennas become the same at certain timing. In the antenna selection diversity using the inverse correlation antenna, the system is selected at the timing at which the reception levels become equal to each other. As a result, a lower limit of the reception level can be guaranteed.
When the delay time of the sea surface reflected wave with respect to the direct wave is increased compared to the symbol interval, the frequency selectivity occurs by changes of levels within the band. In this case, the reception signal needs to be equalized. Since the propagation path characteristics change with time, an adaptive equalizer capable of adaptively updating the equalization coefficients is often used.
Here, an inner configuration example of a general adaptive equalizer will be described.
FIG. 7 shows an inner configuration example of a general adaptive equalizer.
In this example, the tap number of the adaptive equalizer is equal to 3.
The adaptive equalizer of this example includes three registers (Reg) 101-0 to 101-2, three (coefficient variable) multipliers 102-0 to 102-2, and an adder (or accumulator) 103. Further, the adaptive equalizer includes three registers (Reg) 111-0 to 111-2, a coefficient update unit 112, a symbol determination unit 121, a reference signal unit 122, a switch 123, and an adder 124.
The configuration and operation of the adaptive equalizer shown in FIG. 7 is different from that of an embodiment to be described later, e.g., an adaptive equalizer shown in FIG. 2, in that the adaptive equalizer shown in FIG. 7 does not have selection units 22-0 to 22-2 and adjusting units 23-0 to 23-2 shown in FIG. 2 and control signals thereof. This is intended to make the understanding of the adaptive equalizer shown in FIG. 7 clear without unnecessarily limiting the configuration and operation of the adaptive equalizer shown in FIG. 2.
As an example of a prior art, Patent Document 1 describes a configuration example in which when two antennas having a correlation coefficient of −1 receive signals, an inverse correlation in which a reception input (e.g., electric field intensity) of one antenna becomes minimum when a reception input (e.g., an electric field intensity) of the other antenna becomes maximum is obtained (see Patent Document 1).
Further, Patent Document 2 describes an equalizer including a pilot symbol pattern generating circuit for generating a reference signal identical to a symbol pattern (see Patent Document 2).    Patent Document 1: Japanese Patent Application Publication No. 2002-135182    Patent Document 2: Japanese Patent Application Publication No. 2002-344362
For example, the extreme decrease of the reception level caused by changes of the reflection point can be prevented by performing control based on the level changes of the systems by using the inverse correlation antenna.
However, the propagation paths to the antennas have different characteristics. Accordingly, the convergence of the equalization coefficients needs to be performed again. Depending on the required time for the convergence process, the data may be demodulated in a state where the output quality of the equalizer is insufficient. In this case, a bit error ratio is high. When the bit error exceeds the limit of the error correction capability, the quality of the communication service deteriorates.